Java SDK: native Dag definition

### Body

**We should wait until [AIP-85 Dag Importers](https://cwiki.apache.org/confluence/spaces/AIRFLOW/pages/315494137/AIP-85+DAG+importer) pass**.

### Background

The Java SDK (`1.0.0-beta1`) intentionally ships in "Python-stub-Dag mode": Java SDK handlers are registered for Dag/task IDs declared in Python. Higher-level Java Dag authoring was deferred until coordinator-mode execution is reviewed.

### What needs to happen

1. Allow declaring Dags natively in Java-SDK.
2. Revisit bundle ergonomics alongside the authoring interface.

### Acceptance criteria

- A Dag can be authored end to end in Java with no Python stub file.

### Committer

- [x] I acknowledge that I am a maintainer/committer of the Apache Airflow project.